1. Field of the Invention
This invention relates to analog-to-digital (A/D) converters and, more particularly, to such converters of the integrating type.
2. Description of the Prior Art
A/D converters and their companion devices, D/A converters, are essentially interface devices for providing conversion between analog and digital signals. The D/A converter can be thought of as a digitally controlled potentiometer that produces an analog output (either voltage or current generally) that is a normalized fraction of the full-scale setting. An applied reference value is chosen to determine the full-scale output. In an A/D converter, the digital output signal, which may be considered a digital number, depends on the ratio of the quantized input to the full-scale reference.
Because an A/D converter is essentially an interface device, the basic conversion circuitry is adapted to suit particular applications. Such adaptation may include the addition of registers, buffers, a digital clock, and a reference voltage, some or all of which circuits may be provided external to the converter.
A general description of A/D converters and a number of particular circuits thereof are provided in "A User's Handbook of D/A and A/D Converters" by Eugene R. Hnatek (John Wiley & Sons 1976). The A/D converter of the present invention falls in the class of A/D converters described in said publication beginning at page 255 thereof.
Three distinct advantages of an integrating A/D converter are (a) it is a low-cost alternative to A/D converters of the pulse rebalanced type, (b) it achieves high noise immunity and high accuracy by continuous integration of the converter error signal, and (c) it permits simple, low-cost mechanization.
A particular A/D converter of this general type is disclosed in U.S. Pat. No. 3,942,172 of Tucker entitled "Bipolar Mark-Space Analogue-to-Digital Converter". The circuit disclosed therein provides for the summing of a square wave signal at the integrator input with the development of a ramp signal at the output. However, the circuit of the cited patent has an inherent error introduced because the integrator must switch current instantly because of the square wave input. Operation in this fashion causes the circuitry to lose virtual ground for a brief instant, which is a source of error in the developed signal. Moreover, the patented circuitry is limited in its utilization of the positive and negative signal sources to the integrator in that the switches to these sources are separately operated under distinct alternate control signals.
Other A/D converter circuitry of the prior art may be found, for example, in the Wasserman U.S. Pat. No. 3,368,149 entitled "Digital Voltmeter Having a Capacitor Charged by an Unknown Voltage and Discharged by a Known Voltage", the Emmerich U.S. Pat. No. 3,487,204 entitled "High Accuracy Pulse Reset Integrator", the Prozeller U.S. Pat. No. 3,541,446 entitled "Small Signal Analog to Digital Converter with Positive Cancellation of Error Voltages", and The Tippetts U.S. Pat. No. 3,404,857 entitled "Signal Generator for Control Systems". Such prior art indicates the widespread usage and development of analog-to-digital converters for a variety of particular purposes.